The present invention relates to an optical semiconductor amplifier for directly amplifying an optical signal on a transmission path included in an optical fiber communication system.
It has been customary with an optical fiber communication system to amplify an optical signal on a transmission path by converting the optical signal to an electric signal, amplifying the electric signal, and then reconverting the electric signal to an optical signal. The problem with this amplifying procedure is that hardware for the optical-to-electrical and electrical-to-optical signal conversion is indispensable, resulting in a complicated and bulky amplifier. To eliminate this problem, there has been proposed an optical semiconductor amplifier using a GaAs or InP optical semiconductor for directly amplifying light, i.e., without converting it to electricity. The optical semiconductor amplifier has an optical amplifier medium implemented by a semiconductor as small as a semiconductor laser diode and is driven by an injection current. This kind of amplifier is miniature and needs only a simple drive system, although it is inferior to an optical fiber amplifier using an erbium-doped optical fiber in respect of gain and saturation optical output. An optical fiber communication system using a conventional optical semiconductor amplifier determines a gain in terms of injection current by monitoring only the gain of the amplifier. Specifically, the system amplifies light without monitoring light input to and output from the semiconductor amplifier. As a result, light output from the semiconductor amplifier is not stable, causing-the reception level at a receiving section to fluctuate over a broad range.